Partial revert of 3d8b2ec4 to add back DSO_pathbyaddr

Commit 3d8b2ec4

 removed various unused functions. However now we need to
use one of them! This commit resurrects DSO_pathbyaddr(). We're not going to
resurrect the Windows version though because what we need to achieve can be
done a different way on Windows.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(cherry picked from commit cb6ea61c)